Virtual machines
A virtual machine is a program on a computer that emulates another computer!
This is a rather complicated idea so we need to break it down. The word 'virtual' when used in computing means 'pretend'. The word 'emulate' when used in computing means 'works and functions as if it were something else'. So what we are doing is using a particular computer, loading up a special program that then emulates a different kind of computer. Sometimes, we want to use our main computer. Other times, however, we may want to use a different type of computer. We don't need to go out and buy a new, different type of computer. We can buy a much cheaper piece of software that emulates the computer instead.
For example, many people use Apple Macs. However, sometimes, they need to use a program that is only available on a computer running the Windows operating system. What they can do is to load a free or paid-for virtualisation program onto their Apple Mac computer (e.g. VirtualBox, VM Fusion, CrossOver) along with some other software to allow you to select the operating system you want to use called Boot Camp. When they first start their computer, Boot Camp asks them which operating system they want to use. They can then select Apple and use their computer as a normal Apple computer, or Windows, and use their Apple computer as if it were a Windows computer. If they choose Windows, they can then download and run any Windows program they need to use.
Potentially, they could load a range of different virtual machines onto their single computer and they will all run completely independently of each other. On one computer, you could boot into virtual computers running Windows, Linux, Solaris, OS/2, Unix and so on. The downside, however, is that the virtual machines will never be quite as good as having an actual instance of the machine running the operating system because the software has to access the hardware on the host computer indirectly via a third part application (the virtualisation software).